Add support to contextual users in the FlagViewsRelationship plugin

Needs to fix phpcs and phpstan warnings

Thanks for the quick review @ivnish.
PHPCS issues fixed.
Thanks.

Let's create a test with demonstration that it works as expected

Hey @ivnish, I can create a test for this one.

I’ve never written a test specifically for a view relationship. Do you happen to have an example you could share?
I looked at some examples in Drupal core, but they weren’t very helpful.

I did write a test for a view once — I exported the view and installed it in the test environment. Then I created the required entities, loaded the view programmatically, and executed it. But I’m not sure if that’s the best approach for this specific case. What do you think?